On Tuesday, the New York Court of Appeals ruled in a split decision [PDF link] that state police must obtain a warrant before installing a GPS device on an individual's vehicle for tracking purposes.

The decision stems from a case in which Latham police, as part of their investigation of a string of burglaries in 2005, secretly installed a GPS tracking device on the vehicle of an individual named Scott Weaver and tracked his movement for 65 days. Weaver was convicted for robbing one store based at least partly on the GPS tracking, which showed he had traversed that store location at a very slow speed on the evening of the robbery.

Weaver appealed the decision, saying that the GPS tracking had been done without a warrant and constituted a violation of his right against unreasonable search and seizure. In upholding that appeal and granting Weaver a new trial, Chief Judge Jonathan Lippman held that the sort of detailed surveillance enabled by GPS devices mandated judicial oversight. Without it, "the use of these powerful devices presents a significant and, to our minds, unacceptable risk of abuse," Chief Judge Lippman wrote.

On the surface, that decision seemed to be at odds with one made by the Wisconsin Court of Appeals last week in the case of a man who was convicted on stalking charges. The court ruled that the evidence gathered against him using a secretly installed GPS tracking device on his car did not constitute a violation of his Fourth Amendment rights against unreasonable search. In coming to that decision the court avoided directly addressing the issue of whether a warrant is explicitly needed to conduct such GPS tracking in the first place.

But the Wisconsin court's opinion reflected many of the same concerns against the unsupervised used of GPS devices that were expressed by the New York court. Judge Paul Lundsten said the court was "more than a little troubled" by the lack of federal and state laws limiting the government's use of GPS tracking devices. He urged the state's legislature to explore laws imposing limitations on the use of GPS by both government and private actors.

Pam Dixon, executive director of the World Privacy Forum in San Diego, said the decisions show how courts "are looking for limits on the use of GPS" by law enforcement in the absence of any clear federal law on the issue. "Both judges were troubled by the implication of the uses of these devices. Both judges agreed that federal law is needed on whether use of GPS devices constitutes search in the law enforcement context," Dixon said.

The decisions come at a time when an increasing number of law enforcement agencies around the country are using GPS devices to track suspects in investigations. Several states have laws governing the requirements a prosecutor or police would need to meet in order to use a GPS tracking device. The requirements include authorities needing to explain specifically what information they hope to get from the tracking, or showing probable cause or reasonable suspicion.

So far, the courts have appeared to be split on how to handle challenges to such tracking, said John Verdi, senior counsel at the Electronic Privacy Information Center (EPIC) a Washington D.C. advocacy group. "There is no clear Supreme Court guidance on this issue," nor has there by any from any federal courts, Verdi said. As a result, state courts have been left to decide what to do using their own state constitutions, he said.

Some courts have ruled that an individual travelling in a vehicle on a public road should have little reasonable expectation of privacy, Verdi said. He pointed to a 1983 court case in which government agents placed a a pager-like tracking device in a five-gallon drum of chloroform and then followed the vehicle transporting the container using visual surveillance and the tracking device. In that case, the court ruled that the use of the device had only augmented their sensory abilities without violating any of the individual's rights.

In its decision last week, Wisconsin's court of appeals noted that if all that a tracking device did was to reveal vehicle movement, information that could have been obtained by warrantless visual surveillance, no Fourth Amendment violation was involved. Similarly, the police action of attaching a GPS devices to a car and subsequently tracking it does not constitute a search and seizure, the court ruled.

Other courts though have held decidedly less benign views on the use of GPS. Courts in Washington and Oregon have in past cases held that law enforcement authorities require warrants to conduct GPS tracking. In coming to these decisions the judges have argued that the kind of surveillance enabled by GPS devices do not just augment existing abilities, but allows access to a large amount of additional information on an individual, Verdi said.

Chief Judge Lippman wrote that GPS devices are "vastly different and exponentially more sophisticated" devices than pagers or other tracking devices. "Constant, relentless tracking of anything is now not merely possible but entirely practicable."

He said that data retrieved from such devices can open a window into almost every aspect of an individual's life. "What the technology yields and records with breathtaking quality and quantity, is a highly detailed profile, not simply of where we go, but by easy inference, of our associations -- political, religious, amicable and amorous."